Up to now, in an illumination unit such as a head-up display device (hereinafter referred to as “HUD device”), an illumination lens is used for condensing a light from a light source and emitting the light toward an illumination target to illuminate the illumination target.
As one type of the above lens, Patent Document 1 discloses an illumination lens having multiple second lenticular lens surfaces on each of both sides between which multiple first lenticular lens surfaces are sandwiched in a lateral direction. In the disclosure, each of the first lenticular lenses and the second lenticular lenses is shaped into a cylindrical convex lens surface that is twice differentiable in a cross section taken along the lateral direction, and has a curvature different from each other, thereby being capable of exhibiting a light condensing function along each optical axis.
In the illumination lens disclosed in Patent Document 1, the intensity of an emitted light becomes higher on the optical axis of the first lenticular lens surface whereas the intensity of the emitted light becomes lower when the emitted light is spaced apart from the optical axis. Since the above phenomenon occurs in the second lenticular lens surfaces similarly, illuminance unevenness may increase in the illumination derived from the emitted light.